Posting for a friend: .260 Rem AR-10 rifle with 27" bbl and 1-8 twist; national match. Using reformed brass (unspecified). Need loads for the Sierra Match King 142 gr (for 600 yards) and Sierra Match King 107 gr (for 200-300 yards). What works??

You or your friend ought to try

The Highpower Journal
at forums.delphiforums.com/flahighpower/

The people on this forum are highpower shooters who would be glad to provide such data

I haven't found the 107grSMK as accurate in my M700 w/ a 1-8. For the 142grSMK, give H4350, IMR4831 & IMR7828ssc a try. The Lapua 123gr would be a great bullet for both distances. I like H4350 or RL19 under it.